This invention relates to AC voltage sensing and, more particularly, to voltage regulator sensing circuits which sense three-phase voltage, and the sensing method employed by the circuits.
Typical electric generator voltage regulators include a sensing circuit which senses high power AC voltage derived from the generator output and produces a DC output signal proportional to the sensed voltage. The sensing circuit output signal is subtracted from a reference signal to develop an error signal proportional to the difference between the actual generator output and a desired generator output. A voltage regulator feedback control loop is used to adjust the exciter field current of the generator in response to the error signal.
Recent designs of control circuits and amplifiers have improved the operation of voltage regulators to a point where their performance is limited only by the voltage sensing circuit. The two critical requirements for the sensing circuits are accuracy and fast response to transients. In the type of electrical power system typically found on aircraft, constant speed generator systems have used half-wave peak rectifier sensing circuits. Variable speed constant frequency systems have used half-wave average circuits. The use of true RMS or true Mean Square sensing circuits is under consideration.
Half-wave peak rectifier circuits are simple, but their accuracy depends on the waveform crest factor. Such circuits respond quickly to step increases in voltage (at least when the next positive peak occurs) but cannot respond to decreases in voltage faster than the discharge rate of the circuit.
Half-wave average sensing circuits add three half-wave currents to develop a signal with the appearance of the output of a three-phase full-wave rectifier. Such circuits respond to the average of the waveform which gives better regulation, but only sense the positive half-cycles. Half-wave sensing can cause modulation problems.
Both peak and average sensing circuits are inherently electromagnetic-interference susceptible because of their non-linear operation. These circuits include diodes in the signal path which tend to detect the high frequency electromagnetic interference and upset voltage regulation. The diodes must be compensated by adding temperature-dependent resistors to the circuit. Good compensation can only be accomplished at two discrete temperatures, with errors occurring over the rest of the temperature range.
True RMS or Mean Square sensing requires the use of squaring and averaging circuits. Such averaging circuits have inherently slow response, but the regulation is ideal. Such circuits are used in some true RMS meters.
In order to improve voltage regulator performance, a better sensing circuit that combines fast response with the accuracy of true RMS sensing is required.